I didn't see anyone mention on milling a piece of 1080 spring bar. If I need a spring with a lot of taper for the thickness I will soft solder this spring bar to a piece of heavy flat bar and mill the taper first and then polish out the cutter marks. Next heat up to 1500* and form the curve needed. On some springs I had jigs made to get the correct form on V type springs for double shotguns.